Originally Posted by Dave and Julie
The PCM will adjust fuel trim to "optimal" almost instantly. It is constantly adjusting it based on the information it receives from various sensors. We're talking seconds here so that wasn't a factor in your economy.
Very true. Most all the trim comes from adjustment based on calculated fuel ratio. That's entirely off the oxygen sensor. The basic "fuel curves" are based on assumed calibrations of the MAF and fuel pressure and injector capacity. The initial air/fuel mix is derived from that plus a "learned" long term fuel trim value or "LTFT". Small corrections are made using a short term fuel trim value or "STFT" which varies CONSTANTLY. Whenever the STFT value deviates in one direction for an extended period of time, a new LTFT value is calculated, forcing STFT back near zero.
One of the OBD-II codes you can get (which will light your CEL) is one for LTFT out of range. This happens when you have a major problem in calibration, fuel pressure, or sensor malfuction.
As long as the O2 sensor (the first one after the engine) is in good shape, your ratio will not vary much. If it did, you'd get a check engine light.